feat: per-channel hashcash proof-of-work for PRIVMSG anti-spam (#79)

closes #12

## Summary

Implements per-channel hashcash proof-of-work requirement for PRIVMSG as an anti-spam mechanism. Channel operators set a difficulty level via `MODE +H <bits>`, and clients must compute a proof-of-work stamp bound to the channel name and message body before sending.

## Changes

### Database
- Added `hashcash_bits` column to `channels` table (default 0 = no requirement)
- Added `spent_hashcash` table with `stamp_hash` unique key and `created_at` for TTL pruning
- New queries: `GetChannelHashcashBits`, `SetChannelHashcashBits`, `RecordSpentHashcash`, `IsHashcashSpent`, `PruneSpentHashcash`

### Hashcash Validation (`internal/hashcash/channel.go`)
- `ChannelValidator` type for per-channel stamp validation
- `BodyHash()` computes hex-encoded SHA-256 of message body
- `StampHash()` computes deterministic hash of stamp for spent-token key
- `MintChannelStamp()` generates valid stamps (for clients)
- Stamp format: `1:bits:YYMMDD:channel:bodyhash:counter`
- Validates: version, difficulty, date freshness (48h), channel binding, body hash binding, proof-of-work

### Handler Changes (`internal/handlers/api.go`)
- `validateChannelHashcash()` + `verifyChannelStamp()` — checks hashcash on PRIVMSG to protected channels
- `extractHashcashFromMeta()` — parses hashcash stamp from meta JSON
- `applyChannelMode()` / `setHashcashMode()` / `clearHashcashMode()` — MODE +H/-H support
- `queryChannelMode()` — shows +nH in mode query when hashcash is set
- Meta field now passed through the full dispatch chain (dispatchCommand → handlePrivmsg → handleChannelMsg → sendChannelMsg → fanOut → InsertMessage)
- ISUPPORT updated: `CHANMODES=,H,,imnst` (H in type B = parameter when set)

### Replay Prevention
- Spent stamps persisted to SQLite `spent_hashcash` table
- 1-year TTL (per issue requirements)
- Automatic pruning in cleanup loop

### Client Support (`internal/cli/api/hashcash.go`)
- `MintChannelHashcash(bits, channel, body)` — computes stamps for channel messages

### Tests
- **12 unit tests** in `internal/hashcash/channel_test.go`: happy path, wrong channel, wrong body hash, insufficient bits, zero bits skip, bad format, bad version, expired stamp, missing body hash, body hash determinism, stamp hash, mint+validate round-trip
- **10 integration tests** in `internal/handlers/api_test.go`: set mode, query mode, clear mode, reject no stamp, accept valid stamp, reject replayed stamp, no requirement works, invalid bits range, missing bits arg

### README
- Added `+H` to channel modes table
- Added "Per-Channel Hashcash (Anti-Spam)" section with full documentation
- Updated `meta` field description to mention hashcash

## How It Works

1. Channel operator sets requirement: `MODE #general +H 20` (20 bits)
2. Client mints stamp: computes SHA-256 hashcash bound to `#general` + SHA-256(body)
3. Client sends PRIVMSG with `meta.hashcash` field containing the stamp
4. Server validates stamp, checks spent cache, records as spent, relays message
5. Replayed stamps are rejected for 1 year

## Docker Build

`docker build .` passes clean (formatting, linting, all tests).

Co-authored-by: user <user@Mac.lan guest wan>
Co-authored-by: Jeffrey Paul <sneak@noreply.example.org>
Reviewed-on: #79
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

Makefile

@@ -1,4 +1,4 @@
-.PHONY: all build lint fmt fmt-check test check clean run debug docker hooks
+.PHONY: all build lint fmt fmt-check test check clean run debug docker hooks ensure-web-dist
 
 BINARY := neoircd
 VERSION := $(shell git describe --tags --always --dirty 2>/dev/null || echo "dev")
@@ -7,10 +7,21 @@ LDFLAGS := -X main.Version=$(VERSION) -X main.Buildarch=$(BUILDARCH)
 
 all: check build
 
-build:
+# ensure-web-dist creates placeholder files so //go:embed dist/* in
+# web/embed.go resolves without a full Node.js build.  The real SPA is
+# built by the web-builder Docker stage; these placeholders let
+# "make test" and "make build" work outside Docker.
+ensure-web-dist:
+	@if [ ! -d web/dist ]; then \
+		mkdir -p web/dist && \
+		touch web/dist/index.html web/dist/style.css web/dist/app.js && \
+		echo "==> Created placeholder web/dist/ for go:embed"; \
+	fi
+
+build: ensure-web-dist
 	go build -ldflags "$(LDFLAGS)" -o bin/$(BINARY) ./cmd/neoircd
 
-lint:
+lint: ensure-web-dist
 	golangci-lint run --config .golangci.yml ./...
 
 fmt:
@@ -20,7 +31,7 @@ fmt:
 fmt-check:
 	@test -z "$$(gofmt -l .)" || (echo "Files not formatted:" && gofmt -l . && exit 1)
 
-test:
+test: ensure-web-dist
 	go test -timeout 30s -v -race -cover ./...
 
 # check runs all validation without making changes